Fluid level sensors that use capacitors for the primary sensing elements are well known. However, prior devices have numerous drawbacks which result in either excessive cost, decreased accuracy or poor reliability and maintainability over extended periods of use. For example, ring plate capacitor devices are difficult, and hence costly, to manufacture. Detectors that use separate electronic circuits for each capacitor or group of capacitors require time-consuming and labor-intensive calibrations. RF circuits require extensive and precision components that introduce a much greater likelihood that one of those circuits will drift with age. Also, multi-circuit detectors must be reconfigured for each tank or container design. Further, it is well known that tanks or containers change their shape when filled with fluid as compared with their empty state; thus, a tank may have a depth of approximately 7 inches in its near empty state but may expand to a depth of 11 inches when it is filled with a liquid. No known devices exist which compensate for the expansion and contraction of the tank with the volume of liquid it contains. Thus, with a fixed fluid level sensor in the tank, the expansion and contraction introduces an error in the calculation of fluid depth in the tank.
The need, therefore, has long existed for an accurate yet inexpensive apparatus for detecting the level or quantity of fluid in a container and in particular a container which changes shape with the quantity of liquid it contains.